Process of removing surfactant from uncured concrete by flotation

ABSTRACT

Aggregate is separated from sludge consisting of, what is called, &#34;the residual raw concrete&#34; remaining in a raw concrete mixing truck from which raw (uncured) concrete has been discharged, and waste water resulting from washing the mixing truck. The sludge is stored in a pretreatment vessel. Then, air is forced into the sludge to form bubbles. A chemical admixture in the sludge is allowed to adhere to the bubbles, and an admixture coating within the sludge is removed, thereby promoting the hydration of the cement. Subsequently, the resultant sludge is dehydrated to obtain a dehydrated cake of high hardness. Powder formed of the same quality of the cake is spread over the cake, which is then cured for a few hours. The resultant cake is then crushed so that granular aggregate can be formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for disposing raw concretesludge, an apparatus for disposing such sludge, and an apparatus forforming aggregate from such sludge. More particularly, the inventionrelates to a process for disposing raw concrete sludge whereby aggregateis separated from, what is called, "residual raw concrete" remaining ina concrete mixing truck from which a predetermined amount of rawconcrete has been discharged for use, and waste water resulting fromwashing the mixing truck (such residual raw concrete and waste water arehereinafter referred to as "the sludge" in this invention), and thesludge which has thus been separated from the aggregate is disposed. Theinvention also relates to an apparatus for obtaining the aggregate fromthe resultant sludge.

2. Description of Related Art

Conventionally, a chemical admixture (hereinafter simply referred to as"the admixture") is used for raw concrete to improve its quality. Theadmixture is one type of surfactant which acts to reduce the amount ofwater and to circulate together with air. Such an admixture can bedivided into two types. One type of admixture acts to allow cementparticles to be dispersed in water so as to improve hydration effects,thereby promoting the solidification of cement. The other type ofadmixture acts to be adsorbed to the cement particles so as to preventthe initial stage of hydration, thereby delaying the solidification ofcement.

Immediately after being generated, the sludge, such as the residual rawconcrete, is often affected by such an admixture so that the cementparticles are dispersed in water so as to be emulsified. Accordingly,when such sludge is dehydrated on the same day, the filter cloth usedduring dehydration is likely to be quickly clogged with the cementparticles, thus significantly hampering dehydrating characteristics. Inorder to avoid such a problem, the sludge is generally left in anagitation storage vessel (not shown) for approximately ten hours and isthen dehydrated the following day. However, if dehydration is performedafter a lapse of many hours, the hydration reaction of the cement hasgone far ahead, thus deteriorating the agglomeration force of the cementparticles. The resultant cement is soft and difficult to handle, thuscausing pollution if it is set aside.

As described above, the dehydrated cake which has undergone dehydrationthe following day exhibits a weak agglomeration force. Since nodevelopment has been made for measures for recycling such dehydratedcakes, there is no choice but to dispose them as restricted industrialwaste. However, it is difficult to ensure a disposal site, and the costof disposal is consequently very high. In order to solve the aboveproblems, a dehydration process should be performed before the cementloses its agglomeration force immediately after the sludge is generated,thus obtaining the resultant hard cake, which can be recycled, therebysaving a large amount of cost.

However, even though the above problem is overcome to obtain the hardcake of high strength, such cakes are deposited in a conventional mannerso as to result in a massive rock-like shape, which is time-consumingand requires great efforts to destroy.

Further, the direct and immediate processing of the dehydrated cakewithout being cured produces a strong adhesion of the cement water tothe apparatus, which leads to the malfunctioning of the apparatus. Thus,it is difficult to put the foregoing process for disposing raw concretesludge into practical use.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a processfor disposing raw (uncured) concrete sludge and an apparatus fordisposing such sludge, thereby obtaining dehydrated cake of highstrength.

Another object of the present invention is to provide an apparatus whichis capable of forming easily-handled granular aggregate withoutpermitting the dehydrated cake to stick to the apparatus, or the like.

In order to achieve the above objects, the present invention provides aprocess for disposing raw concrete sludge employed whereby a dehydratedcake of high strength can be obtained from the sludge, the processcomprising the steps of: supplying raw concrete sludge to a pretreatmentvessel so as to store the sludge therein; supplying air into the sludgeso as to form a plurality of bubbles; allowing an admixture contained inthe sludge to adhere to the plurality of bubbles; promoting thehydration of cement due to a synergistic effect of removing theplurality of bubbles from the sludge, the bubbles having the admixtureadhering thereto, and destroying an admixture coating within the sludgeby the bubbles; and dehydrating the sludge.

The pretreatment vessel has a capacity corresponding to the disposalamount of the sludge in one cycle of dehydration. Such a vessel has anagitation function and also allows the cement to have a durationrequired for the hydration reaction caused by the bubbles.

The plurality of bubbles having the admixture adhering thereto may beremoved by collecting the bubbles which have reached the top surface ofthe sludge.

The sludge may be supplied to the pretreatment vessel by a sand pumpwhich is vertically movable within a sludge vessel for storing thesludge, the sludge vessel being arranged corresponding to thepretreatment vessel.

The sludge vessel may receive the sludge supplied from a separator whichseparates aggregate from the raw concrete, the separator being arrangedcorresponding to the sludge vessel.

The present invention also provides an apparatus for disposing rawconcrete sludge comprising: a pretreatment vessel for storing rawconcrete sludge; a sludge supply device for supplying the sludge to thepretreatment vessel; an air supply device for supplying air into thesludge stored in the pretreatment vessel so as to form a plurality ofbubbles; a removal device for removing the plurality of bubbles formedby the air supply device from the sludge; and a dehydrator fordehydrating the resultant sludge having the bubbles removed therefrom.

The sludge vessel may communicate with a separator which separatesaggregate from the raw concrete, the separator being arrangedcorresponding to the sludge vessel.

The pretreatment vessel may comprise an agitator for agitating thesludge.

The removal device may be a collecting mechanism for collecting thebubbles floating on the top surface of the sludge.

The air supply device may be a hermetic hollow member having a pluralityof holes and may be communicated with the agitator and also with anpressurizing air supply device.

The sludge supply device may comprise a sand pump arranged to bevertically movable within the sludge vessel for storing the sludge.

Among the foregoing admixtures, agents for dispersing cement particlesin water so as to improve the hydration action, that is, what is called,"the water reducing agents", generally have an anionic type and anon-ionic type.

A lignin-type water reducing agent of the anionic type is mixed into thesludge so as to cause mutual repulsion among cement particles, which arethen dispersed in water, thereby allowing a large number of cementparticles to react with water and facilitating a sufficient hydration.

On the other hand, a hydroxycarboxylic acid water-reducing agent isadsorbed to the cement particles so as to prevent the initial stage ofthe hydration of the cement, thereby inhibiting the gelation anddelaying the solidification of the cement.

As is seen from the foregoing description, attention has been given tothe fact that the admixtures permit cement particles to be dispersed inwater, while it acts to cover the cement particles therewith but notcombined with the cement particles.

The above-noted technique of mixing and removing an admixture isemployed as follows. Fine air particles are mixed into the sludge so asto produce bubbles, and accordingly, an admixture floats to the surfaceof the sludge with the bubbles. The removal of the bubbles has a doubleeffect of removing the bubbles and destroying a residual admixturecoating covering the cement in the sludge.

Further, the following technique is employed in the present inventionwhereby a solution containing bubbles is reused by mixing it intokneading water for raw concrete and whereby filtration water resultingfrom the dehydration is reused for disposing the residual concrete andwashing the inside of the concrete mixing truck.

The following technique is also employed whereby the sludge istransferred to the pretreatment vessel while vertically moving thesuction inlet at all times, thereby enhancing uniform concentration ofthe sludge.

The present invention further provides a dehydrated cake disposingapparatus comprising: a cutter for slicing a dehydrated cake, the cutterbeing arranged below a dehydrator for dehydrating raw concrete sludge; atransfer device for transferring the dehydrated cake, the device beingarranged below the cutter; and a powder spreader arranged to correspondto the position of the dehydrated cake located by the transfer device.Aggregate can be formed from the raw concrete sludge with such adehydrated cake disposing apparatus.

The transfer device may be adapted to move forward for two strokes andto move backward for one stroke.

The present invention still further provides a process for disposing adehydrated cake, comprising the steps of: dropping a dehydrated cakeobtained by dehydrating sludge in a dehydration process onto a beltconveyer; slicing the dehydrated cake by a cutter in a raised state onthe belt conveyer so as to form it in bar-like pieces; and spreadingcake powder over both surfaces of the dehydrated cake by a spreader, thecake powder being formed of the same quality as the dehydrated cake sothat the bar-like pieces are prevented from melting and sicking to eachother.

Such coating means spreads the cake powder on the belt conveyer byreciprocating the conveyer. The belt conveyer then moves backward, andthe dehydrated cake is sliced and laid on the belt conveyer. The beltconveyer is again transferred under the spreader so that the cake powdercan be spread over the dehydrated cake.

The present invention still further provides an apparatus for formingaggregate from raw concrete sludge, comprising: a storage vessel fortemporarily storing a dehydrated cake which has been dehydrated andcoated with cake powder therein; a ribbon screw conveyer for crushingand transferring the cake; a cutter for cutting the dehydrated cake, thecutter being arranged at the rear end of the ribbon screw conveyer; apowder spreader arranged in a stage subsequent to the cutter; and atrommel arranged in a stage subsequent to the powder spreader.

The dehydrated cake formed as described above and coated with the cakepowder is temporarily cured in a storage vessel for a few hours so as toproceed with the hydration to such an extent That the cake does notstick to the apparatus or the other components. In such a state, theresultant cake is transferred while being crushed and is rolled into agenerally spherical shape. The cake pieces in a rock-like shape are cutagain by the cutter arranged in a stage subsequent to the ribbon screwconveyer. The resultant cake granules are transferred to the trommelwhile being coated with the cake powder by the spreader so that they canbe prevented from sticking to each other. The cake granules are furthersubjected to a transformation into a spherical shape and the removal offine particles therefrom. The resultant aggregate is then classified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrative of a process for disposing rawconcrete sludge, an apparatus for disposing such sludge, and anapparatus for forming aggregate from the resultant sludge according toan embodiment of the present invention;

FIG. 2 illustrates a detailed portion of the apparatuses shown in FIG.1;

FIG. 3 illustrates a process step of the operation performed by theaggregate forming apparatus shown in FIG. 2;

FIG. 4 illustrates a process step of the operation performed by theaggregate forming apparatus shown in FIG. 2;

FIG. 5 illustrates a process step of the operation performed by theaggregate forming apparatus shown in FIG. 2;

FIG. 6 illustrates a process step of the operation performed by theaggregate forming apparatus shown in FIG. 2;

FIG. 7 illustrates a process step of the operation performed by theaggregate forming apparatus shown in FIG. 2;

FIG. 8 illustrates a process step of the operation performed by theaggregate forming apparatus shown in FIG. 2;

FIG. 9 is a front view of a dehydrated-cake granulating apparatus whichis arranged stage subsequent to the apparatus shown in FIG. 2;

FIG. 10 is a top view of the apparatus shown in FIG. 9;

FIG. 11 is a sectional view along the line XI--XI of FIG. 9; and

FIG. 12 is a sectional view along the line XII--XII of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A description will now be given of an embodiment of process steps of thepresent invention with reference to the drawings.

Referring to FIG. 1, raw concrete brought from a construction site andsludge 2 resulting from washing the inside of the mixing truck 1 aredischarged into a mixing truck gutter 3 so as to flow into a trommel 4(a separator) which is arranged downstream of the gutter 3. Sand 5 andravel 6 are separated from the sludge 2 in the trommel 4. The sludge 2is supplied to a sludge vessel 7 and is further supplied to apretreatment vessel 9 by a sand pump 8.

The sand pump 8 hangs from a lift 10 in order to suck the sludge 2 byturns from the respective water-depth layers of the sludge vessel 7. Anagitator 11 is provided for the pretreatment vessel 9. An air diffuser12 injects air into the sludge 2 within the pretreatment vessel 9 and acollector 14 allows the sludge 2 to flow into a U-shaped groove 15 so asto remove bubbles 13 floating on the surface of water. The aqueoussolution containing the bubbles 13 is collected in a tank 18 and isdiverted for the use of kneading water 19 for the raw concrete.Filtration water 17 from a dehydrator 16 is further recycled to thestart of the process by a filtration water pump 20 so as to be reusedfor various purposes, such as disposing, what is called, "the residualconcrete" and washing the concrete mixing truck 1. It can also be reusedas washing water for the trommel 4.

In the present invention, fine air particles are mixed into the sludge 2as described above, and accordingly, an admixture accompanies thebubbles so as to be floated therewith. The removal of the bubbles has adouble effect of removing the bubbles and destroying the residualadmixture. The functions of the admixture are thus removed so as topromote the mutual agglomeration of cement particles in the sludge sothat the agglomerated cement is able to be dehydrated after a lapse of asuitable duration required for the hydration reaction. The aqueoussolution which has liquefied the bubbles is reused by mixing it into thekneading water for the raw concrete.

The water content of the thus-obtained dehydrated cake conspicuouslydiffers from that of a conventional cake which is dehydrated thefollowing day. Originally, the water content required for the cement tobe dehydrated and solidified should be thirty-odd % (twenty % of waterrequired for hydration and ten-odd % of water required foragglomeration). However, if dehydration is performed the following day,which process is generally rendered, the hydration reaction for thecement has gone far ahead, which cement has already been emulsified,thus accordingly making it difficult to be dehydrated. The content ofresidual water in such cement is generally high, as much as 40% orhigher, thereby providing very low compressive strength for theresultant dehydrated cake.

In contrast thereto, the water content of the dehydrated cake obtainedby the process of the present invention is low, such as twenty-odd %.Even though the water incorporated into the cement by the hydrationreaction by the time dehydration is performed is taken into account, thewater in the dehydrated cake of the present invention is considerablyreduced in comparison with that of a conventional dehydrated cake, whichratio is approximately 50:50 of water to cement, prepared by a typicalprocess. As a result, the resultant dehydrated cake has remarkablecompressive strength as indicated in the following Table 1.

The present invention is thus largely characterized by the followingfeatures.

The properties of an admixture which is easily adsorbed into the air isutilized. There are provided novel means for destroying an admixturecoating created within the sludge by bubbles. Giving attention to thefact that dehydration characteristics of the cement sludge water dependson the duration for the agitation and the hydration reaction, apretreatment vessel is arranged to act to adjust such duration.

The sand pump, which is a lift-type means for feeding the sludge fromthe sludge vessel to the pretreatment vessel, inhibits a disparity ofthe concentration of the sludge depending on the cycle of thedehydration process, thereby making the advantages obtained in thepretreatment step more effective.

Table 1 shows the measurements of the compressive strength of thedehydrated cake obtained by the process of the present invention and theconditions for such measurements.

    __________________________________________________________________________    Strength test on dehydrated cake                                                                 Set up the same conditions as those                                                             Raw concrete purchased for this                                               experi-                                  according to calculation table made by                                                           for concrete brought back from a                                                                ment Supplied from a raw concrete                                             site                                     company A          construction site of company A                             Obtained from cake sample C                                                                      Conditions for concrete mixed in a                                                              Conditions for concrete mixing and                          raw concrete site before performing                                                             producing                                                   a hydration process                                        __________________________________________________________________________           Sample                                                                            Age                                                                              Strength                                                                           Concrete materials were mixed in                                                                Symbol depending on the type of                                               concrete                                 Sampled date                                                                         No. (day)                                                                            kg/cm.sup.2                                                                        the raw concrete site, and at the time                                                                      Standard                     Nov. 27, 1993                                                                        1   7  408  after a lapse of 3 hours, washing                                                               Nominal strength (SL)                                                                     240 kg/cm.sup.2              Nov. 27, 1993                                                                        2   7  329  water was mixed to the concrete                                                                 Slump       18 cm                        Apr. 27, 1994                                                                        3   9  491  materials to form a sludge having a                                                             Ratio of water to cement                                                                  54%                          Apr. 27, 1994                                                                        4   9  493  specific gravity of 1.15.                                                                       Admixture   high perfor-                                    The sludge was intermittently mance Chupol                                    agitated for 30 minutes.      HP-8                                            Then, the resultant sludge was dehy-                                                            (Standard mixing)                                           drated according to a dehydration                                                               A mixture of cement, water and                                                admix-                                                      process of the present invention.                                                               ture. Aggregate is excluded for                                               con-                                                        Water content of the cake samples                                                               venience's sake.                                                1   18%                                                                       2   22%                                                                       3   20%                                                                       4   20%                                                __________________________________________________________________________

As is clearly understood from the foregoing detailed description, theconcentration of the sludge is averaged so that the process steps inevery cycle can be simplified. Consequently, whenever the sludge igenerated, it can be directly dehydrated, thereby significantlyshortening the duration required for the hydration reaction of thecement, and enabling a reduction in the water content of the resultantdehydrated cake. In addition to the above-mentioned advantages, thefiltration water resulting from dehydrating the sludge from which hashad the admixture removed is used as washing water, thereby giving theadvantage of decreasing the amount of calcium dissolved in the water.This significantly improves the strength of the dehydrated cake, whichis thus reusable, thereby leading to a considerable cost reduction ofthe dehydrated cake which otherwise should be disposed as industrialwaste and to an improvement in cost efficiency. This further results ina contribution to the prevention of pollution in local areas.

A subsequent step followed by the removal of the cake which has beendehydrated by the dehydrator 16 is performed by an apparatus illustratedin FIG. 2. An upward-slope belt conveyer 26 is arranged downward acake-receiving hopper 23 so that a dehydrated cake 25 can be droppedunder the belt conveyer 26 from each filtration chamber 24 via thecake-receiving hopper 23. The belt conveyer 26 is adapted to operate bythe following conditions. One stroke is determined as a length 32occupied by the dehydrated cake 25 dropped onto the belt conveyer 26.The belt conveyer 26 moves forward for two strokes and backward for onestroke, which movement is determined as one cycle. The belt conveyer 26thus continuously performs the movement of one cycle every time thedehydrated cake 25 drops onto the belt conveyer 26. A spreader 27 forspreading cake powder is arranged upward of the belt conveyer 26 ontowhich the dehydrated cake 25 is dropped. The spreader 27 is operated incooperation with the movement of the belt conveyer 26 so as to spreadthe cake powder all over the width of the belt to the limit in abar-like shape. Two stages of dampers 28 and 29 are further provided forthe spreader 27 in order to spread a predetermined amount of powder inevery one cycle of the movement of the belt conveyer 26. The amountcontained in the two stages of dampers 28 and 29 are adapted to matchthe amount of powder supply in one cycle. A cutter 30 is arranged at thebottom of the cake-receiving hopper 23 to slice the dehydrated cake 25dropped onto the belt conveyer 26 into a plurality of pieces.

The operation of the foregoing apparatus will now be explained. Uponcompletion of dehydration, a movable plate 30 moves back and thedehydrator 16 temporarily stops when the applying pressure reachesapproximately from 0 to 10 kg. Sensing this stoppage of the dehydrator16, the belt conveyer 26 moves in the reverse direction for 3 or 4seconds so as to be stopped (See FIG. 3). This reverse movement of thebelt conveyer 26 is performed to remove the dripping filtration waterupon completion of dehydration. Subsequently, the belt conveyer 26performs one cycle of the movement, that is, it moves forward for twostrokes and backward for one stroke (See FIGS. 4 and 5). In cooperationwith this movement of the belt conveyer 26, the damper 29 arranged atthe outlet of the powder spreader 27 is sure to be gradually opened. Theduration for opening the damper 29 is adapted to match the durationrequired for one cycle of the belt conveyer 26. The damper 29 is fullyopened and is successively closed. At this time, the dehydrated cake 25is ready for dropping (See FIG. 5). Then, the process goes into a stepof dropping the cake 25 (See FIGS. 6, 7 and 8). The belt conveyer 26first goes ahead for one cycle as described above, and at this time,there is no cake 25 loaded on the belt conveyer 26. However, the powder31 which has gone ahead of the cake 25 is spread into a storage vessel33 before the cake 25 drops from the forward end of the belt conveyer26.

Upon completion of the cake dropping step, a subsequent dehydration stepfollows, and then, the cake-dropping step is again performed. Thisprocess is repeated until the sludge generated for the same day iscompletely eliminated, thus completing a daily dehydration operation.

An explanation will now be given how the apparatus works to drop thecake 25. The first cake 25 is dropped onto the belt conveyer 26 via thehopper 24. The cutter 30, extended by the cylinder 33, vertically slicesthe cake 25 into bar-like pieces, which are then cut apart whenevernecessary by the reciprocation of the cutter 30. Subsequently, the beltconveyer 26 moves for one cycle, while the cake powder is spread (SeeFIGS. 7 and 8). Then, a subsequent cake 25 is dropped onto he beltconveyer 26. The cutter 30 further reciprocates so as to allow thesubsequent cake 25 to be cut apart and the cake 25 is laid on the beltconveyer 26. Such a step is repeated.

According to the present invention, the cake 25 is dropped onto the beltconveyer 26 from the filtration chamber 24 via the hopper 23 in such away that the dropping distance is contained as small as possible so asto allow the cake 25 to retain its original shape. The cake 25 isvertically sliced by the cutter 30 for several times, and at the sametime, the powder 31 is spread over the cake 25 from upward. That is, asufficient amount of powder formed of the same material as the cake 25is spread so as to prevent the cake 25 from melting, therebysignificantly reducing the occurrences of sticking of the cake 25 to theapparatus. This also gives rise to the advantage of easily crushing thecake 25 into small pieces in the subsequent crushing process step. Thepowder-coated dehydrated cake produced according to the foregoingprocess is indicated by reference numeral 36.

FIGS. 9-12 show a crushing apparatus 44 for crushing the dehydrated cake36. The crushing apparatus 44 is provided with a temporary storagevessel 37 for temporarily storing the powder-coated dehydrated cake 36.A double-door cake-receiving plate 38 for longitudinally opening outwardis further arranged within the temporary storage vessel 37, therebycontrolling the amount of the cakes 36 that are dropped. Thecake-receiving plate 38 may be modified to be a slide-type plate so asto be slidably opened and closed. A ribbon screw conveyer 39 having alarge movement of pitch is arranged below the cake-receiving plate 38. Acutter 42 having a plurality of teeth is arranged at the rear end of theribbon screw conveyer 39, and a powder-spreading mechanism 43 is furtherarranged above the cutter 42 to spread cake powder from the upwarddirection. A trommel 45 is provided alongside the crushing apparatus 44so that the cake 36 passing through the cutter 42 can be transferred tothe trommel 45 by a bucket conveyer 46 or a screw conveyer (not shown).

The operations of the crushing apparatus 44 and the trommel 45 will nowbe described. The cake-receiving plate 38 is arranged to prevent thepowder-coated dehydrated cakes 36 from dropping into the temporarystorage chamber 37 at one time, otherwise a ribbon screw casing 47 wouldbe completely filled with the dehydrated cakes 36. A certain time isallowed to leave the cakes 36 after being dropped into the temporarystorage chamber 37 so that the progress of hardening the cake 36 due tohydration can be adjusted. The cake 36 is transferred by the ribbonscrew conveyer 39 while being repeatedly agitated slowly little bylittle. The cake 36 is finely cut by the cutter 42. The cake powder isspread by the spreading mechanism 43 to prevent the cake 36 from meltingand sticking to the apparatus. Further, the trommel 45 again mixes thecake 36 with the cake powder and further granulates and classifies theresultant cake 36, thereby automatically screening the cake particles bythe particle size. Thus, granular cake suitable for recycling can beobtained.

As will be clearly understood from the foregoing description, thepresent invention offers the following advantages.

After the cake 36 provided with an agglomeration force obtained by theimmediate dehydration process is stored in the temporary storage vessel37, the progress of hardening the cake 36 caused by the hydrationreaction is adjusted after a lapse of a few hours for leaving theresultant cake 36 in the storage vessel 37. Subsequently, the cake 36 isgradually supplied little by little by the ribbon screw conveyer 39,while controlling the opening amount of the receiving plate 38. The cake36 is thus crushed into small particles, which are then rolled, therebyforming the crushed cake 36 into a spherical shape. The thus-shaped cakeparticles are coated with the cake powder to prevent the particles fromsticking to each other. Then, the resultant cake 36 is further subjectedto the granulation and classification by the trommel 45, thus obtainingthe granular cake suitable for recycling.

What is claimed is:
 1. A process for removing and destroying surfactantfrom uncured concrete sludge, comprising the steps of:supplying uncuredconcrete sludge to a pretreatment vessel so as to store said sludgetherein; supplying air into said sludge so as to form a plurality ofbubbles; allowing most of any surfactant contained in said sludge toadhere to said plurality of bubbles; removing said plurality of bubblesfrom said sludge, said bubbles having said surfactant adhering thereto;and destroying surfactant coatings in said sludge by said bubbles, saidsurfactant coatings covering cement particles in said sludge, whereinthe concrete sludge remaining after the surfactant coatings have beendestroyed has a composition that allows newly-generated surfaces of saidcement particles to hydrate and form coarse particles so as to prevent afilter cloth from clogging.
 2. The process for removing surfactant fromuncured concrete sludge according to claim 1, wherein said plurality ofbubbles having said surfactant adhering thereto are removed bycollecting said bubbles which have reached the top surface of saidsludge when said pretreatment vessel is filled with said sludge.
 3. Theprocess for removing surfactant from uncured concrete sludge accordingto claim 1, wherein said sludge is supplied to said pretreatment vesselby a pump which is vertically movable within a sludge vessel for storingsaid sludge, said sludge vessel being arranged corresponding to saidpretreatment vessel.